Inductively coupled drive module for electrically powered models

ABSTRACT

An inductively coupled drive module for small scale model vehicles, the module including an electric motor having at least one output shaft having an axis of rotation; and at least one inductive coupling, the inductive coupling having: a body which includes a drive shaft which is coaxial with said output shaft, an opening to permit a first end portion of said output shaft to extend within the body, bearings located within the body to permit the body to rotate about said axis but independently of rotation of said output shaft, and a member mounted on said output shaft for rotation therewith and within a cavity formed in the body, the member being: (a) made from or includes magnetic material, or (b) made from or includes electrically conductive material; and wherein the body is: (c) made from or includes electrically conductive material, (when the member is made from or includes magnetic material), or (d) made from or includes magnetic material (when the member is made from or includes electrically conductive material), the arrangement being such that when the electric motor rotates and its output shaft rotates, the member rotates within the body so that induced currents will flow in the body or member which currents will react the magnetic material in the member or mounting body and cause a torque to be applied to the body to thereby cause rotation thereof and consequential rotation of the drive shaft.

More particularly the invention relates to an Inductively Coupled DriveModule [ICDM] designed for application to electrically powered modelrailway locomotives. It will be appreciated that the ICDM could be alsoused in other models such as land vehicles (cars, trucks, militaryvehicles or the like) which incorporate electric motors for providingthe movement of the models.

The ICDM is specifically designed for application to “Small Scale”models. “Small Scale” covers the range of model scales between 160:1 (1prototype ft=2 mm) and 32:1 (1 prototype ft=10 mm). The most popularmodel railway scale worldwide, generically known as HO scale, has ascale ratio of 87:1 [1 prototype ft=3.5 mm]. The detailed description ofthe ICDM relates to this scale, however ICDM's constructed in accordancewith this invention cater for all small scale models.

Installation of an ICDM in a model provides truly prototypical modeloperation that is unable to be achieved with any presently availableelectric drive mechanism.

All electric-motor driven models are presently fitted with “directconnected” drive systems, which means that the electric driving motor ispermanently connected through some form of fixed-ratio reduction gearset to the model's driving wheels. Substantially all “small scale” modelrailway locomotives are powered by 12 volt DC motors, where 0 V dc giveszero speed and 12 V dc gives maximum torque and full speed.

The gear sets are assembled to provide the required gear reductioneither from a selection of spur gears, or a combination of spur gearsand a worm drive.

The resultant combination of motor power, model speed and tractiveeffort is therefore always a compromise design, which provides a presetand fixed set of operating conditions that prevent the model fromoperating in a way which properly replicates the operation of theprototype.

Operation of a model railway locomotive in prototypical mode requiresthat the model replicates, as closely as possible, the operationalcharacteristics of a prototype locomotive powered by a diesel or petrolengine. Prototype operation is similar in many respects to the operationof a motor car with automatic transmission, in that the engine power iscontinually adjusted by the driver modifying the engine speed (byaccelerator adjustment) to meet the varying requirements of tractiveeffort and speed. For example, starting to move a car uphill when towinga load such as a boat on a trailer requires much higher tractive effortthan starting to move the same car with no load downhill.

All prototype diesel or petrol engine railroad locomotives incorporatesome form of variable power transfer mechanisms between the drivingengine and the wheel sets to enable them to start smoothly and operateat varying speeds with varying loads. Prototype engines are usuallystarted in a no load mode to ensure that engines are protected frominitial overload and that correct operating conditions are establishedbefore working loads are applied.

Electric motor driven model railway locomotives are not currently ableto accurately replicate prototypical variable torque powercharacteristics. In known electric motor driven model railwaylocomotives, the preset, fixed mechanical coupling between the drivingmotor and the driven wheel or wheels of the locomotive results in theelectric motor speed to locomotive speed ratio being fixed, thusprecluding variable torque and preventing accurate replication ofprototypical operation.

In accordance with the invention, the problems of non-prototypicaloperation of a model associated with the prior art can eliminated byreplacing the fixed mechanical coupling between the drive motor and thedriving wheels with an ICDM.

According to the present invention there is provided an inductivelycoupled drive module for small scale model vehicles, the moduleincluding an electric motor having at least one output shaft having anaxis of rotation; and at least one inductive coupling, the inductivecoupling having:

-   -   a body which includes a drive shaft which is coaxial with said        output shaft,    -   an opening to permit a first end portion of said output shaft to        extend within the body,    -   bearings located within the body to permit the body to rotate        about said axis but independently of rotation of said output        shaft, and    -   a member mounted on said output shaft for rotation therewith and        within a cavity formed in the body, the member being:        -   (a) made from or includes magnetic material, or        -   (b) made from or includes electrically conductive material;            and    -   wherein the body is:        -   (c) made from or includes electrically conductive material,            (when the member is made from or includes magnetic            material), or        -   (d) made from or includes magnetic material (when the member            is made from or includes electrically conductive material),            the arrangement being such that when the electric motor            rotates and its output shaft rotates, the member rotates            within the body so that induced currents will flow in the            body or member which currents will react with the magnetic            material in the member or mounting body and cause a torque            to be applied to the body to thereby cause rotation thereof            and consequential rotation of the drive shaft.

Preferably the module includes first and second of said inductivecoupling.

The invention also provides small scale model locomotive including

-   -   a chassis,    -   wheels mounted on the chassis,    -   an inductively coupled drive module as defined above mounted on        the chassis, and    -   means for coupling said drive shaft of said inductive coupling        to at least one of said wheels.

The invention also provides a method of operating a model vehicle havinga chassis, wheels and electric motor and a transmission for coupling anoutput shaft of the motor, the method including the steps of:

-   -   providing an inductive coupling in said transmission, the        coupling including an electrically inductive component and a        magnetic component, and    -   selecting the size, location, conductivity and/or magnetic        strength of said components so that there is a predetermined        coupling efficiency between said parts and the motion of the        model substantially replicates the motion of the prototype        thereof.

Preferably, the coupling efficiency of the ICDM can be adjusted toenable its effective use in a variety prototype models in differentscales and model weights.

Preferably, further the module includes an arrangement for selecting theinductive coupling transfer characteristics for model frame sizes, wherethe frame sizes are determined by the physical size constraints of themodel scale ratios.

The output from each inductive coupling is normally connected to thedriving wheels of the model through standard spur and/or worm/worm gearsets. With the ICDM installed in a model locomotive there is no longer adirect mechanical connection between the drive motor and the drivingwheels and prototypical operation of the model is now possible dependingupon the design parameters of the ICDM motor and integrated inductivecouplings.

The invention will now be further described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view of a model locomotive chassis incorporatingan ICDM constructed in accordance with this invention;

FIG. 2 a is a side view partly in section of an ICDM constructed inaccordance with the invention;

FIG. 2 b is an end view of the IDCM;

FIG. 3 is a cross-sectional view through one of the two inductivecouplings of the invention; and

FIG. 4 is a cross-sectional view along line 24-24.

FIG. 1 shows a model vehicle 26 such as a model locomotive, with thebodywork removed. The locomotive includes a chassis 5, front wheels 6and rear wheels 7. An ICDM 1 is mounted on the chassis 5 and has forwardand rear output shafts 10 and 11.

The ICDM 1 is constructed in accordance with the invention and comprisesa motor 2 with forward and rear inductive couplings 3 and 4 integralwith motor 2 output shafts 14 and 15. The vehicle 26 includes reductiongear sets 8 and 9 having input shafts 12 and 13 respectively. The ICDMoutput shafts 10 and 11 are coupled to the gear set input shafts 12 and13 respectively. The gear sets 8 and 9 normally include internalbearings. Therefore the input shafts 12 and 13 do not require separatebearings because they are coupled at one end to the gear sets 8 and 9and at the other end to the inductive couplings 3 and 4 all of whichhave their own bearings. The shafts 12 and 13 would normally includesplined couplings (now shown) intermediate of their length so as toallow for limited elongation thereof which is required for rotation ofthe front and rear wheel 6 and 7 during cornering. The ICDM 1 could beprovided with a mounting plate (not shown) upon which the motor 2 ismounted in order to facilitate connection of the ICDM 1 to the chassis5. In most cases however a modeler would normally mount the ICDM usingadhesive tape or the like so that a mounting plate would not normally berequired.

FIG. 2 a shows a side view of the ICDM 1 with inductive couplings 3 and4 integral with motor 2 and the inductive coupling 3 shown in crosssection. FIGS. 3 and 4 show the structure of the inductive coupling 3 inmore detail. The inductive couplings 3 and 4 are identical and thereforeonly one of them needs to be described in detail.

The coupling 3 includes cylindrical parts 17 and 18 which are joined attheir circumference to form a mounting body 16 for the components.Within the body 16 is an air cavity 21 within which is located a discmagnet 20 attached to the extended shaft 14. Two miniature ball bearings22 and 23 are mounted in the body 16 and are attached to motor shaft 14and enable the mounting body 16 to rotate freely about motor shaft 14.The cylindrical part 17 is connected to the end of the forward shaft 10so that the shafts 10 and 14 can rotate coaxially but independently ofone another. A hole 25 in the cylindrical part 18 allows the shaft 14 topass into the interior of the body 16. The output shaft 10 provides aconnection between the coupling 3 and the driving wheels 6 via gear setinput shaft 12 and reduction gear set 8. The cylindrical parts 17 and 18are preferably made from electrically conductive material and arepreferably relatively dense so that the body 16 functions as a flywheel.Brass or copper is a suitable material for the parts 17 and 18.

In use, the motor 2 output rotates by applying a variable DC voltage ofup to 12 volts across its two power input terminations (not shown). Thepolarity of the applied motor voltage determines the rotationaldirection of motor 2. The voltage level determines the speed of rotationof motor 2. Rotation (in either direction) of motor 2 causes rotation ofmagnet 20 in unison therewith. The permanent magnet 20 is magnetizedwith alternate poles as shown in FIG. 4. The rotation of magnet 20causes induced currents to flow in the mounting body 16 which is locatedadjacent thereto causing a reactive force which produces consequentialrotation of the body 16. The output shaft 10 rotates with the body 16and causes rotation of wheels 6 via input shaft 12 and gear set 8. It isnoted that the coupling 3 operates as described for either direction ofrotation of motor 2, thus the directional movement of the model 26 isdetermined by the polarity of the power supply to motor 2.

The operating characteristics and physical size of motor 2 together withoperating characteristics and size of couplings 3 and 4 are matched toprovide a desired range of ICDM outputs for each model size and weightwithin each model scale.

Using the most common scale of “HO” as an example only, motor 2 is aminiature flat can of 16 mm wide×20 mm high×25 mm long, with a maximumspeed of 19,000 RPM and a maximum stall torque of 100 gmf·cm. Because ofthe miniature sizes of the inductive couplings required to enable anICDM to be fitted to small scale models, motor rotational speeds ofbetween 12,000 and 15,000 RPM are normally required to transfer therequired driving power and torque to the wheel sets of the model. Theminiature inductive couplings 3 and 4 are preferably 15 mm diameter and15 mm long with an air cavity 21 of 13 mm diameter and 5 mm long. Theoverall dimensions of ICDM 1 for HO scale fit within a volume frame of61 mm long, 16 mm wide and 20 mm high.

The output power of ICDM 1 is determined by motor 2 input voltage andthe selection of magnet 20 characteristics which determine the powertransfer function of couplings 3 and 4. It will be appreciated thatusing the same specifications for magnet 20, but varying its diameterand width within the limits of the coupling air cavity 21 will alter theeffective air gap 19 between the magnet surfaces and the walls of theair cavity 21, thus providing a range of maximum ICDM output powerlevels for the same power input to motor 2. The couplings 3 and 4 arenormally fitted with magnets of the same size and specification toensure that power to wheel sets 6 and 7 are equal. All magnets arepreferably of rare earth materials and have a strength of BH equal toabout 38 MGOe.

The following details provide typical examples of the sizes of themagnet 20 to provide differing ICDM maximum power outputs when fullvoltages are applied to the motor 2.

EXAMPLE 1

A small light mainline diesel HO scale model locomotive requires amaximum torque of about 20 gmf·cm to operate prototypically under allconditions. This torque level is obtained with magnet 20 dimensions of12 mm diameter and 3.5 mm wide; at a motor 2 rotational speed of 11,200RPM. The air gap 19 is thus 0.5 mm in the radial direction and 0.75 mmin the axial direction.

EXAMPLE 2

A large heavy mainline diesel HO scale model locomotive requires amaximum torque of about 40 gmf·cm to operate prototypically under allconditions. This torque level is obtained with magnet 20 dimensions of12.75 mm diameter and 4.5 mm wide; at a motor 2 rotational speed of14,600 RPM. The air gap 19 is thus 0.125 mm in the radial direction and0.25 mm in the axial direction.

Smaller scales and larger scales constructed in accordance with thisinvention utilize motors and couplings to suit the volume frame sizeavailable for each particular scale.

In operation, a model locomotive fitted with an ICDM will start movingwhen the ICDM output exceeds the static frictional forces. These forcesare dependent on the model weight and bearing and wheel friction. Powerto the locomotive wheel sets is controlled by the voltage applied to theICDM motor and the power transfer function of the ICDM inductivecouplings. This control voltage is varied by the operator using athrottle control. The operator must increase the throttle position toincrease the voltage to the ICDM motor to a level where the ICDM outputpower exceeds the restraining forces, mainly attributable to staticfriction. Once motion of the model has started, the restraining forceswill change according the movement of the models, and thus the throttleposition should be adjusted to maintain a required motion. Prototypicaloperation of the model can therefore be substantially replicated.

Many modifications will be apparent to those skilled in the art withoutdeparting from the spirit and scope of this invention. These may includea single-ended ICDM employing only one coupling, which is suitable fordriving models with a single wheel set; or differing motor-magnetmatching combinations for specific model applications.

It will also be appreciated that the module can be made to function ifthe positions of the magnet and the conductive material are reversed. Inother words, the disc 20 would be made from or include electricallyconductive material and the mounting body 16 would be formed from orinclude permanent magnet material so as to have at least one pair ofpoles. In this arrangement if the disc 20 is rotated induced currentsflowing therein will generate magnetic forces which in turn will reactwith the permanent magnet to cause rotation of the body 16.

Further, in the preferred embodiment described above, the electric motorhas a single shaft the ends of which form the output shafts 14 and 15which extend into the inductive couplings 3 and 4. It would be possibleto construct the module with separate shaft components which are joinedtogether to form a single shaft although the unitary shaft as describedabove is preferred.

1. An inductively coupled drive module for small scale model vehicles,the module including an electric motor having at least one output shafthaving an axis of rotation; and at least one inductive coupling, theinductive coupling having: a body which includes a drive shaft which iscoaxial with said output shaft, an opening to permit a first end portionof said output shaft to extend within the body, bearings located withinthe body to permit the body to rotate about said axis but independentlyof rotation of said output shaft, and a member mounted on said outputshaft for rotation therewith and within a cavity formed in the body, themember being: (a) made from or includes magnetic material, or (b) madefrom or includes electrically conductive material; and wherein the bodyis: (c) made from or includes electrically conductive material (when themember is made from or includes magnetic material), or (d) made from orincludes magnetic material (when the member is made from or includeselectrically conductive material), the arrangement being such that whenthe electric motor rotates and its output shaft rotates, the memberrotates within the body so that induced currents will flow in the bodyor member which currents will react the magnetic material in the memberor mounting body and cause a torque to be applied to the body to therebycause rotation thereof and consequential rotation of the drive shaft. 2.A drive module as claimed in claim 1 including first and second of saidinductive couplings, and wherein the output shaft has a second endportion which passes through the opening in the body of the secondinductive coupling.
 3. The drive module as claimed in claim 2 whereinsaid first and second portions are integral with said output shaft.
 4. Adrive module as claimed in claim 2 wherein each member is made frommagnetic material which is magnetized so as to have at least two poles.5. A drive module as claimed in claim 2 wherein each member is a discconcentrically mounted on respective end portions of said output shaft.6. A drive module as claimed in claim 5 wherein air gaps are definedbetween the discs and said cavities.
 7. A drive module as claimed inclaim 5 wherein the air gaps include radial and axial air gaps.
 8. Adrive module as claimed in claim 7 wherein the radial air gaps are inthe range from 0.125 to 0.5 mm.
 9. A drive module as claimed in claim 7wherein the axial air gaps are in the range from 0.25 to 0.75 mm.
 10. Adrive module as claimed in claim 1 wherein the size of the module issuch that it can be located within a cuboid having the followingdimensions: length 61 mm, width 16 mm and height 20 mm.
 11. A smallscale model locomotive including a chassis, wheels mounted on thechassis, an inductively coupled drive module as claimed in claim 1mounted on the chasses, and means for coupling said drive shaft of saidinductive coupling to at least one of said wheels.
 12. A method ofoperating a model vehicle having a chassis, wheels and electric motorand a transmission for coupling an output shaft of the motor, the methodincluding the steps of: providing an inductive coupling in saidtransmission, the coupling including an electrically inductive componentand a magnetic component, and selecting the size, location, conductivityand/or magnetic strength of said components so that there is apredetermined coupling efficiency between said parts and the motion ofthe model substantially replicates the motion of the prototype thereof.13. An inductively coupled drive module for small scale model vehicles,the module including an electric motor having at least one output shafthaving an axis of rotation; and at least one inductive coupling, theinductive coupling having: a first shaft having a first coupling membermounted thereon for rotation therewith; a second shaft having a secondcoupling member mounted thereon for rotation therewith; mounting meansfor mounting the first and second shafts for rotation co-axially withsaid axis of rotation and the first shaft being constrained to rotate inunison with the output shaft of the motor; and wherein one of the firstand second coupling members is made from or includes magnetic materialhaving a magnetic field and the other of the first and second couplingmembers is made from or includes electrically conductive materialwhereby rotation of the motor causes rotation of the first shaft wherebythere is relative movement of said electrically conductive material andsaid magnetic field causing in use currents to be induced in theconductive material which produce a rotational torque on said secondshaft.
 14. A drive module as claimed in claim 13 including first andsecond of said inductive couplings.
 15. The drive module as claimed inclaim 14 wherein the drive shaft of the motor is integral with the firstshaft of the first and second inductive couplings.
 16. An inductivelycoupled drive module for small scale model vehicles, the module beingadapted for coupling to an electric motor having a motor output shaft,the module having at least one output shaft having an axis of rotation;and at least one inductive coupling, the inductive coupling having: afirst shaft having a first coupling member mounted thereon for rotationtherewith; a second shaft having a second coupling member mountedthereon for rotation therewith; mounting means for mounting the firstand second shafts for rotation co-axially with said axis of rotation andthe first shaft being constrained, in use, to rotate in unison with themotor output shaft; and wherein one of the first and second couplingmembers is made from or includes magnetic material having a magneticfield and the other of the first and second coupling members is madefrom or includes electrically conductive material whereby rotation ofthe motor causes rotation of the first shaft whereby there is relativemovement of said electrically conductive material and said magneticfield causing in use currents to be induced in the conductive materialwhich produce a rotational torque on said second shaft.